Page 597 - Instrumentation Reference Book 3E

Ultrasonics 579

-4z.k
L
A

aE--

Driving L
Absorber current
k L
Time Frequency
Figure 24.22 Pulseshape andassociated spectra.
are shown in Figure 24.22. The types of output
are:
(1) Single d.c. pulse with reactangular shape;
Individual crystals (2) Oscillating pulse with rectangular envelope
and carrier frequency so;
(3) D.c. pulse with an exponential rise and
decay;
(4) Oscillating pulse with exponential rise and
decay and carrier frequency jb.
From these results it can be seen that the main
lobe of the spectrum contains most ofthe spectral
energy, and its width is inversely proportional to
the pulse duration. In order to obtain a broad
ultrasonic spectrum of large amplitude the excita-
I tion pulse must have as large an amplitude and as
Compression //: short a duration as possible. In practice, a com-
/' ; promise is required, since there is a limit to the
/I
breakdown strength of the transducer and the
output voltage of the pulse generator. Electronic
(d) equipment used in pulse spectroscopy (Figure
Figure 24.20 Special probe construction. (a) Focused
probe; (b) variable-angle probe: (c) mosaic probe; 24.23) differs considerably from that for fre-
(d) angled compression pr'obe. quency modulation, including a time gate for
selecting the ultrasonic signals to be analyzed,
and so allowing observations in the time and
frequency domains.
A pulse frequency-modulation technique can
also be used which combines both the previous
procedures (Figure 24.23). The time gate opens
and closes periodically at a rate considerably
higher than the frequency sweep rate, hence
breaking the frequency sweep signals into a series
of pulses. This technique has been used princi-
pally for determining transducer response char-
ransducer 1
3 acteristics.
Specimen
ransducer 2 9 Applications of ultrasonic spectroscopg
24.4.5
Y
Oscilloscope
t 24.4.5. I Transducer response
It is known that the results of conventional ukra-
sonic inspection can vary considerably from
Figure 24.21 Frequency-modulation spectroscope transducer to transducer even if element size and